The invention relates to laser output couplers and more particularly to a mirror output coupler. Tunable lasers are of special interest, because by using wavelength sensitive elements within the laser cavity, a virtual continuum of laser wavelengths can be generated. This is of great practical value as it extends the possibility of laser illumination to virtually any wavelength of interest. In general, for tunable lasers, the optical gain within the laser medium is a function of wavelength. Of the two mirrors which form a laser cavity, the back mirror is usually of maximum reflectance, while the output mirror has a transmission usually picked to optimize laser output.
It is also well known in the laser art that the laser output is a function of optical gain, mirror reflection coefficients and system losses. Because of this functional dependence, for a given set of laser parameters, and for a particular wavelength, there is an optimum transmission coefficient for the output mirror which maximizes output. In terms of a simplified laser model, this means that optimum coupling is achieved as a balance of two considerations: on the one hand, it is desirable to have as low a transmission as possible (highest reflectivity) on the output mirror in order to build up as much energy as possible in the light waves traveling back and forth in the laser, and on the other hand, the higher the transmission of the output mirror, the higher the fraction of light which escapes the cavity as output for each reflection.
If the output power of a laser with fixed gain in the laser medium, fixed losses, and a perfect back mirror (.about.100% reflectivity) is plotted against transmission of the output, mirror, a curve is obtained which starts near zero, rises to a peak level at optimum transmission, and falls again towards zero. Lasers are usually designed and operated near the maximum of such a curve. However, since the gain of a tunable laser typically varies as a function of wavelength, it is not possible in the case of tunable lasers to use one transmission value to optimize output coupling for different wavelengths.
Solutions to this problem have been varied. In some cases, workers have used a series of mirrors to cover different wavelength ranges. In other cases, broad band mirrors have been used with reflectivity characteristics chosen to attempt to match the needed reflectivity at most wavelengths. These methods have met with some degree of success, but do not provide continuously variable output coupling for each wavelength.
At least two previous methods to provide continuously variable output couplers are known. In the first method, described in U.S. Pat. No. 3,448,404, a Brewster window and a total reflector at 90 degrees are combined to provide variable output coupling from inside the laser cavity. In the second method, described in U.S. Pat. No. 3,624,546, two totally reflecting mirrors at 90 degrees to each other are used to define a variable gap with a resultant variable output coupling. These two methods suffer from certain disadvantages. The first is that they both require extra optical components in addition to the two normally used for cavity resonators. Further, in the first method, the output coupler is inside the laser cavity and requires an extension of the distance between the cavity resonators. For very short pulse lasers, it is important that the laser resonators be as close as possible to each other. Also, in the second method, the output coupling hole is not circularly symmetric, which leads to a non-circularly symmetric output beam.
Accordingly, it is an object of the present invention to devise a continuously variable output coupler with a minimum number of optical components, and which is consistent with a short cavity and gives an essentially circularly symmetric output beam.